Generally, the packaging operation presently being commercially used to package frozen poultry necessitates that the packaging materials exhibit special properties. The typical packaging operation includes the following steps:
1. Dropping the bird into a bag fabricated from a shrinkable film;
2. Evacuating the bag;
3. Clamping the neck of the bag;
4. Transporting the bird with the entire weight thereof being supported by the neck of the bag;
5. Subjecting the bag to a temperature of from about 90.degree. C. to about 95.degree. C. for a period of time up to about 7 seconds to shrink the bag tightly around the bird;
6. Quick freezing and then storing the packaged bird at temperatures as low as about -40.degree. C.; and
7. Transporting the packaged bird and thereby possibly subjecting the packaged bird to continuous abrasion and impact against adjacent packaged birds during shipping.
The packaging operation requires the shrinkable film to possess the following general properties:
1. Sufficient tensile strength to support the weight of the bird;
2. Shrinkage and reduction in the area of the film at a temperature of from about 90.degree. C. to about 95.degree. C., sufficient to conform the film to the irregular shape of the bird. Generally at least 25% linear size reduction (unrestrained) in the direction transverse to the machine direction of the film has been required, usually at least 29%, and preferably at least 31%.
3. Shrink force at a temperature of from about 90.degree. C. to about 95.degree. C., sufficient to pull the wings of a bird tightly towards the body, and sufficient residual shrink force to maintain a tight wrap around the bird;
4. Sufficient seal strength for a heat seal, so that after the film has been fabricated into a bag by a conventional heat sealing operation, there will be no seal failure during a subsequent shrink packaging operation at a temperature of from about 90.degree. C. to about 95.degree. C.; and
5. Impact strength and abrasion resistance at about -40.degree. C., sufficient to protect the bird during freezing, shipping and consumer handling.
In addition to the above enumerated general requirements, the film must possess dimensional stability and non-tacky surfaces, and it should have processing characteristics which enable thickness control and high production efficiency at reasonable manufacturing speeds.
In the prior art, several film materials have been either used or proposed for packaging frozen poultry. For example, U.S. Pat. No. 3,555,604 to Pahlke discloses a process for producing bioriented low density polyethylene in the form of a shrinkable thermoplastic film. In this process, a tubing of polyethylene material defines an isolated film bubble maintained by simple nip rollers, and the bubble is subjected to heat and radial expansion by internal pressure near the draw point of the tubing, that is, the point at which the polyethylene material is at or just below its softening point. This process is generally referred to as the "double bubble" method.
The biaxial stretching of low density polyethylene can also be carried out by using an irradiated film and stretching it by methods well known in the art, to obtain a film having satisfactory melt strength and shrinkage values at a temperature of about 90.degree. C. This film, however, is relatively expensive and has marginal heat sealing properties because of the loss of thermoplastic characteristics due to the crosslinking of the polymer chains induced by the irradiation. Irradiation also inhibits the reprocessing of waste film. The U.S. Pat. No. 3,365,520 to Foster et al describes the xylene solubility test for distinguishing between irradiated and non-irradiated polyethylene.
The aforementioned U.S. Pat. No. 3,365,520 also discloses a blend of an ionomer and an ethylene homopolymer or copolymer for producing a biaxially stretched film having exceptional shrinkage and shrink force at a temperature of about 90.degree. C., and having good processing characteristics. Unfortunately, the film has very poor heat sealing properties for use in packaging poultry. While heat seal strength at room temperature is adequate, the strength at about 95.degree. C. is extremely low.
The U.S. Pat. No. 3,900,635 to Funderburk et al discloses a multilayer film including a blend of an ionomer and ethylene homopolymers or copolymers, and another layer of ethylene homopolymers or copolymers. The film exhibits adequate shrinkage at about 90.degree. C. and good seal strength. The film, however, does not possess certain other properties desirable for poultry packaging. For example, the abrasion resistance of this film is not significantly better than the abrasion resistance of a conventional biaxially stretched low density polyethylene single layer film.
Although each of the known films meets many of the requirements for packaging applications such as the packaging of frozen poultry, the need still exists for improved heat shrinkable films.